feat(train): the sync engine and Train's first models (ADR 0030, D14)

Train's server syncs with serverpod_offline_sync as Learn's does, and
holds three synced tables, every payload sealed on the devices:
content_key (the person's data key, sealed to the account), workout (its
type, times, name and totals, changed when edited) and workout_series
(chunks of a workout's samples, written once). A person's space goes
with them, with everything in it.

The migration only adds: the three tables, each indexed on spaceId and
cascading with its space (a series also with its workout), and the sync
engine's own tables.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/train/halkyon_train_client/lib/migrations/migration_registry.dart

@@ -0,0 +1,19 @@
+/// AUTOGENERATED BY SERVERPOD create-migration - DO NOT EDIT BY HAND
+/// Merging: resolve registry conflicts the same as migration_registry.txt.
+library;
+
+import 'package:serverpod_database/serverpod_database.dart';
+
+part '20261028101012641-workouts/migration.dart';
+
+/// Migration registry for the client-side database.
+class MigrationRegistry {
+  /// Ordered list of all client-side database migrations.
+  /// New migrations are always appended at the end.
+  static final List<MigrationVersionSql> migrations = [
+    _Migration20261028101012641_workouts(),
+  ];
+
+  /// List of all client-side database migration versions.
+  static List<String> get versions => migrations.map((m) => m.version).toList();
+}